docs: def _compute_C1_term(C, dists):
      """
      Return C1 coeffs as function of Rrup as proposed by
      Rodriguez-Marek et al (2013)
      The C1 coeff are used to compute the single station sigma
      """
  
      c1_dists = np.zeros_like(dists)
      idx = dists < C['Rc11']
      c1_dists[idx] = C['phi_11']
      idx = (dists >= C['Rc11']) & (dists <= C['Rc21'])
      c1_dists[idx] = C['phi_11'] + (C['phi_21'] - C['phi_11']) * \
          ((dists[idx] - C['Rc11']) / (C['Rc21'] - C['Rc11']))
      idx = dists > C['Rc21']
      c1_dists[idx] = C['phi_21']
      return c1_dists